A car wash service has decided to discontinue its repeat customer

This question is a bit challenging because it's difficult to identify the type of question. Looking at the question itself - "Which of the following is a reason the argument is most vulnerable to criticism" - we can see that the question is dealing in some way with weakening the argument. It's basically asking, "why would you be able to weaken this argument?". In order to weaken the argument, you need to identify an assumption (the unstated portions of the argument that must be true in order for the conclusion to be valid) and then attack the assumption. So, the key to this question is really to identify the assumption.

The assumptions will sit between the conclusion and the premises. Here the conclusion is that the car wash company made a good move by removing is couponing program. The premises are 1) the workers are paid by the car so that 5th car wash includes costs but no revenues and 2) the success of the program required many of the 6th, "cost-only" car washes, which would cause the overall company's profits to decline.

What would we have to assume in order for the conclusion (it's a good move to stop the coupon program) to remain valid? Since the argument is solely focused on expenses here and profits are revenue minus expenses we should be thinking about assumptions that are related to revenues.

A) Remember that assumptions sit in the space between conclusion and premise. Adding a different program may be a good move for the company, but it has no connection to the removal of the current couponing program.
B) This one is a bit tricky. Here the answer is shifting meaning on us. The premise refers to the declining profits but this answer choice is referring to the overall profits of the company. The GMAT likes to play these little meaning shift games on incorrect answers - be careful.
C) This is not an accurate statement. Since labor costs are variable (each wash costs money), the operating costs would be impacted by removing the program.
D) This one tries to make you think that if the problem were smaller we could change our mind about the effectiveness of the program. However, the argument is based on the variable costs of each wash, so whether we have a huge number of washes or a small number of washes, we would have declining profits either way.
E) Finally something strong about revenues. Here we have a necessary assumption that is placed in a weakening statement. In order to believe the conclusion that removing the coupons is a good move, we first have to believe that there were no benefits (particularly related to revenues) achieved from the program between the 1st and 5th washes. As the OA states, the program may have induced many customers to purchase additional car washes in order to hit the 6th wash. So, even though that 6th wash causes a reduction in company, the profits would have been greatly increased by the those additional purchases. This statement makes the argument vulnerable to criticism.

KW